For the growth of III-N materials (i.e. GaN, AlN and InN and their alloys) on 50, 100, 150, 200 mm, or even larger Si substrates, it is often discovered that although the tensile strain due to thermal mismatch is carefully compensated by a strain engineered buffer, the wafer is fragile during further process handling. The fragility manifests itself e.g. in the GaN-on-Si wafers breaking into large pieces with fairly high frequency during steps involving thermal processing (e.g. anneals, high temperature film deposition/etching etc.) and mechanical handling (e.g. chemo-mechanical polishing, wafer bonding etc.).
For example, it was noticed that the fragility of 200 mm diameter 725 μm thick GaN-on-Si wafers was caused primarily by the formation of slip-lines in the Si substrate during the substrate annealing step e.g. before the Low Temperature (LT)-AlN deposition. This is believed to be due to the presence of vertical and radial temperature variations across the 200 mm Si substrate. The Si crystal slip takes place if the local stress exceeds the yield strength at the annealing temperature (1050° C.) prior to the LT-AlN growth. There are two possible major sources of stress on the Si substrate in MOCVD growth. They are the contact stresses between the wafer and the point at which it contacts the susceptor, and the thermal stress due to temperature non-uniformity in the vertical and radial directions across the wafer. The slip lines originate from the edge of the wafer and propagate toward the center of the wafer. Minimizing radial temperature differences across the 200 mm Si wafer during growth through the optimization of heater zone settings is one key way to reduce slip formation and wafer fragility, but it is not possible to fully eliminate vertical temperature differences through the wafer due to heat only being supplied to the back-side of the wafer. Thus, in almost all cases, wafer fragility remains an issue due to the high growth temperatures involved in III-nitride on Si epitaxy.
Embodiments of the present invention seek to address at least one of the above problems.